Defects in the articular surfaces of the knee joint, especially in young active individuals, are currently a focus of interest by orthopedic surgeons. Damage to cartilage which protects joints can result from either physical injure (e.g. osteochondral fracture, secondary damage due to cruciate ligament injury) or from disease (e.g. osteoarthritis, rheumatoid arthritis, aseptic necrosis, osteochondritis dissecans). Osteoarthritis results from general wear and tear of joints and is common in the elderly. Rheumatoid arthritis is an inflammatory condition which results in the destruction of cartilage. It is thought to be, at least in part, an autoimmune disease with suffers having a genetic predisposition to the disease. Orthopedic prevention/repair of damaged joints is a significant burden on the medical profession both in terms of expense and time spent treating patients.
Drug intervention to ameliorate or prevent the onset of cartilage loss are available but do have significant disadvantages.
As an alternative to drug intervention, thus avoiding undesirable side effects, orthopedic surgery is available in order to repair defects and prevent articular damage, thereby leading to serious degenerative changes in the joint. Such changes may result in the need for a total knee replacement which is particularly undesirable in young active individuals with a long life expectancy. If the lifetime of the implant is less than that of the patient, a revision procedure may be necessary. Preferably, such revision procedures are to be avoided, having regard to inconvenience to the patient. Furthermore implant revision procedures are both lengthy and very costly. The use of surgical techniques to repair/replace damaged tissue in joints often requires the removal and donation of healthy tissue to replace the damaged or diseased tissue. There are three sources of donating tissue used in tissue engineering of this type:
i) autograft: tissue is removed from an area of the patient remote from the region to be repaired and grafted to the damaged region to effect the repair;
ii) allograft: tissue is removed from a donating individual, for example a cadaver, and transplanted to the damaged region; and
iii) xenograft: tissue is harvested from another animal species, for example a pig, and placed over the damaged area.
Autografts can be problematic due to the limited availability of suitable tissue and the added trauma to the patient during removal of the tissue from another part of the body to the damaged area. Allografts are limited by immunological reactivity of the host, availability of suitable donor tissue and the problem of transfer of infective agents. Xenografts are even more problematic due to the severe immunological reactivity.
Various techniques for cartilage repair are either in limited current use or under, development but publicly disclosed. The Osteochondral Autogenous Transplant System (OATS) of Arthrx Inc. is arguably the most widely used method. Osteochondral plugs are harvested from a healthy donor and, more particularly, from a site which is claimed to be ‘non-weight-bearing’. These plugs are transplanted into the site of the chondral defect. This procedure has been applied primarily in the knee joint.
However, there are no donor sites in the knee with cartilage of a comparable thickness to that of the deficient site which can be described as ‘non-weight-bearing’ areas. The sulcus terminalis, a frequently used site for harvesting such grafts, is in direct contact with the lateral meniscus at the position of full knee extension, and is therefore a weight-bearing site.
Furthermore, harvesting a large osteochondral plug from the sulcus terminalis may cause the lateral meniscus to become loose and thus impair its load-bearing function. As a result, all the tibio-femoral loads would be transmitted onto the small area of direct contact between the femur and tibia. The resultant stresses could be as high as those arising after meniscectomy with its consequential degenerative changes in the cartilage of the tibial plateau. Such changes have always been regarded as precursors to osteoarthritis.
While the OATS method provides a reasonable technique, including good instrumentation, for transplanting live autogenous grafts for repair of defects in cartilage, it involves introducing potentially damaging effects at other sites with the serious disadvantages discussed above. In addition, harvesting a plug from a donor site creates a new damage in the knee articular surface. For this reason, OATS would not be suitable for the repair of large defects. The use of OATS for small repairs would probably limit the magnitude of the problem discussed above, but it would also limit the indication for using this technique.
The technique known as Autogenous Chondrocyte Implants (ACI) of Genzyme Inc. is a conceptually elegant approach which is gaining popularity, but still in limited use. The procedure is intended for repair of small as well as large irregular defects, and is achieved in a two stage surgery. In the first stage, chondrocytes (cartilage cells) are harvested from the patient and cultured in suspension. In the second stage of the operative procedure, cartilage residue is cleared from the repair site. The site is then covered with a piece of periosteal tissue which is sutured to the perimeter of the repair area. The chondrocytes are then injected into the repair site using a hypodermic syringe, puncturing the periosteum with the needle of the syringe. In a variation of this procedure, the periosteal tissue is applied to the repair site in the first stage of the operation to ensure that, by the time the chondrocytes are due to be injected, an adequate seal has formed between the tissue and the perimeter of the cartilage. There is a high probability of the chondrocytes escaping through the hole of the hypodermic needle in either version of the procedure.
A further problem with the second version of the procedure is the probability of tissue adhesions occurring between the periosteal tissue and the bottom of the repair site.
This procedure does not have an established rate of success and the quality of cartilage in the repair site is questionable. As with the OATS method, this procedure is not minimally invasive. Further, it is an extremely costly procedure. It is also a disadvantage that it requires two operative procedures although the first stage is less invasive as it can be performed arthroscopically.
A procedure proposed by Smith & Nephew involves the production of cartilage discs formed by allogenic chondrocyte culture on an absorbable textile fabric. The discs are grown in the laboratory, the allogenic chondrocytes being cultured on a matrix of a non-woven mesh of a bioabsorbable material, typically polyglycolic acid. When this procedure is completed, the disc is supplied for implantation at the repair site.
An advantage of this method is that it does not involve damage to an intact healthy chondral site since the method uses allogenic sources. Furthermore the procedure is completed in a one stage operation.
The discs can be made in different sizes but there must be a limit to the size of the defect which can be repaired with a loose disc which is merely placed on the repair site. The implant could move freely in the joint. It could wrinkle under the influence of tangential forces and could be completely damaged as a result. This problem would be exacerbated by a low compressive modulus of the material.
A further disadvantage with this method is that the material, being an allograft, runs the risk of viral infection, for example, the HIV virus. Although a small risk, this is an inherent problem with any allograft.
A further problem to be anticipated with this type of graft is the compressive modulus of the material. It may be quite low and the material might be in need of mechanical conditioning (a time consuming and costly process) to achieve a modulus compatible with that of cartilage of the surrounding area.
The Depuy cartilage repair system is a disc of non-woven fabric made of bioabsorbable material that has a hard substrate which enables the implant to be attached to the bone. The shape of the disc allows repair of damaged areas of irregular shapes by using a plurality of discs in a close-packed array. The disadvantages with this system are that the use of too many adjacent hexagonal discs will result in much damage to the bone substrate, and, further the technique may require considerable skill and its application may also be time consuming.